With an increase of demands for large-capacity data transmission, research and developments being conducted on a digital coherent optical transmission method which enables a high-speed transmission with a single wavelength light. In addition, as a technology of enhancing a nonlinear resistance in the digital coherent optical transmission method, digital sub-channel multiplexing (DSCM), which multiplexes a plurality of sub-channels having different center frequencies by a digital signal processing to optical-modulate the sub-channels, has been studied and developed (see, e.g., Non-Patent Document 1).
According to this technology, one signal may be constituted by multiplexing a plurality of sub-channels having a lower symbol rate than the symbol rate of a wavelength division multiplexing (WDM) transmission (see, e.g., Patent Document 1) of an optical signal. As a result, a nonlinear degradation is reduced. In addition, Non-Patent Documents 2 and 3 disclose a technology relating to a channel monitoring means in a dense wavelength division multiplexing (DWDM) system.
Related technologies are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2014-217053.
Related technologies are disclosed in, for example, Meng Qiu, “Digital subcarrier multiplexing for fiber nonlinearity mitigation in coherent optical communication systems,” Optics Express, 2014, Vol. 22, Issue 15, p. 18770-18777 22, Issue 15, p. 18770-18777 (Non-Patent Document 1); Takahito Tanimura et al, “Superimposition and Detection of Frequency Modulated Tone for Light Path Tracing Employing Digital Signal Processing and Optical Filter,” OFC/NFOFC 2012, OW4G.4 (Non-Patent Document 2); and Takahito Tanimura et al, “In-band FSK Supervisory Signaling between Adaptive Optical Transceivers Employing Digital Signal Processing,” ECOC 2011, We.7.A.6 (Non-Patent Document 3).